1. Field of the Invention
This invention relates to a gamma camera for providing an image representing the radioisotope (which is hereinafter referred to as RI) distribution in a test subject previously dosed with radioisotope, and more particularly to an improvement of a position detector for calculating a radiation ray incident position.
2. Description of the Related Art
The general construction of the conventional gamma camera is shown in FIG. 1. The gamma camera includes a scintillator 1 for receiving a radiation ray (gamma ray) radiated from a test subject previously dosed with radioisotope and emitting scintillation light corresponding to the received gamma ray. The scintillator 1 is optically coupled to the light receiving surface of a plurality of photomultipliers 2 for converting the received scintillation light into an electrical signal. The scintillator 1 and photomultipliers 2 constitute a gamma ray detector 9 in cooperation with a collimator and a light guide which are not shown in FIG. 1.
Output signals of the gamma ray detector 9, that is, signals from the photomultipliers 2 are amplified by pre-amplifiers 3 and then supplied to a position detector 4. The position detector 4 derives position data x and y representing the incident position or emitting position of the radiation ray receiving surface of the scintillator 1, on which the gamma ray is incident or from which the scintillation light is emitted.
For example, the position detector 4 is formed of a weighting circuit using a resistor matrix so as to derive four parameters X.sup.-, X.sup.+, Y.sup.- and Y.sup.+ in the X-Y orthogonal coordinate system having the center of the radiation ray receiving surface of the scintillator 1 as an origin thereof as well as a parameter Zdiv. Then, the position detector 4 determines radiation ray incident position data x and y based on these parameters as follows: EQU x=(X.sup.+ -X.sup.-)/Zdiv (1) EQU y=(Y.sup.+ -Y.sup.-)/Zdiv (2)
Output signals of the photomultipliers 2 are also supplied to an additional amplifier 6 via variable resistors 5, added together therein, and then supplied to a pulse height analyzer 7. The pulse height analyzer 7 derives radiation ray energy data Zspc and supplies the same to a display unit 8. When the energy level is larger than an upper limit value or smaller than a lower limit value, the pulse height analyzer 7 supplies an unblanking signal UNB to the display unit 8. The display unit 8 displays an image representing the RI distribution in the test subject according to the position data x and y, the unblanking signal UNB, and the radiation energy data Zspc.
A problem occurring in the conventional gamma camera described above is that a plurality of gamma rays are received at the same time by the scintillator 1. The probability that a plurality of gamma rays are incident on the scintillator 1 at the same time becomes higher as the mean incident rate becomes higher. In this case, since the gamma ray incident position on the scintillator 1 is derived by the position detector 4 which is formed by the weighting circuit using the resistor matrix, the incident position is derived as the central position or the mean point of a plurality of incident positions. That is, the incident position is erroneously derived. Since the erroneous calculation may occur more frequently in the central portion of the detector 9, a profile becomes a convex portion in the center of the detector and the contrast of the image becomes irregular in the case of the RI diagnosis of high counting rate.
Further, since the cross section of each of the photomultipliers 2 is circular, a large number of photomultipliers may be arranged in a hexagonal configuration so as to minimize the space between the photomultipliers. FIG. 2 shows an example of the arrangement of the photomultipliers of the conventional gamma camera. In this example, 61 photomultipliers are closely arranged in a hexagonal configuration.
In this case, if the weighted values W1 to W5 given to the first to fifth (No. 1 to No. 5) photomultipliers on the first row of the hexagonal array are set such that W1=-2, W2=-1, W3=0, W4=+1 and W5=+2 and a gamma ray is incident on the position indicated by an arrow of broken line as shown in FIG. 3, then the incident position data x in the X-axis direction can be obtained as follows: ##EQU1##
where A1 to A5 indicate output pulse heights of the No. 1 to No. 5 photomultipliers.
FIG. 3 shows only the photomultipliers arranged along the X-axis direction but the incident position data y in the Y-axis direction ca be derived in the same manner.
In this case, when the gamma ray is incident on a position inside the photomultiplier array and all the six photomultipliers surrounding the incident position are present, no problem occurs. However, when the gamma ray is incident on the peripheral portion of the array and not all the six photomultipliers surrounding the incident position are present, the pulse height of the outer photomultiplier is 0 so that the calculation for deriving the incident position will be effected based on unbalanced factors. As a result, the position derived is slightly shifted in a inward direction with respect to the actual position. If an RI distribution image is provided based on such position data, it may be distorted on the peripheral portion thereof.